Hi Wes,
What you say is true if the characteristic impedance of the line is equal to
the system reference impedance (Zo). In other words, if your are changing
the length of a 50 ohm feedline in a 50 ohms system, then the only thing
that can improve VSWR would be additional loss from the line (e.g. the VSWR
will be constant along the length of a lossless line). Thus, system bandwidth
will be the same at any point along the length of the lossless line whose Zo
matches the system Zo.
If the line Zo is different than the system Zo (e.g. the classic case of 75 ohm
CATV cable used in a 50 ohm system), then the direction of the load reactance
change versus frequency can have a significant bearing on the system
bandwidth. In this case, the length of the line also plays a significant role.
73 de Mike, W4EF............
----- Original Message -----
From: "Wes Attaway" <wes@attawayinterests.com>
To: <K3BU@aol.com>; <towertalk@contesting.com>
Sent: Wednesday, October 10, 2001 6:23 PM
Subject: RE: [TowerTalk] Re: Force12
>
> These opinions re changing bandwidth of an antenna by adding or subtracting
> feedline are the silliest things I have heard in a long time. Tom, and many
> others, are correct in that making bandwidth broader by adding feedline is
> only introducing loss. Saying it has done something magical is on about the
> same level as saying you can change the SWR by playing with feedline
> lengths.
>
> Ideas about correcting impedance mismatches, and etc, beg the point because
> nothing has really changed at the antenna. You may fool a meter at the
> input to the line, but you have not really changed anything about the
> antenna.
>
> I remember an old quote from QST, long long ago when I was much younger.
> The author's name escapes me at the moment, but he was a well-known QST
> editor. His statement was very simple: "The only thing that changes SWR on
> a feedline is the load at the end of the line". This is pretty basic, and
> the facts have not been repealed either by modern electronics or by
> modeling.
>
> SWR is not directly related to bandwidth, of course, but I refer to it here
> only to emphasize my point about weird misconceptions. One thing modeling
> will definitely do is give you a pretty good idea of how the bandwidth curve
> will look for a given antenna. If you can go outside and change the
> feedline length and create a better curve, and think something good has
> happened, then you had better get the ARRL Anenna Handbook out and start
> reading.
>
> Wes - N5WA
>
> -----Original Message-----
> From: owner-towertalk@contesting.com
> [mailto:owner-towertalk@contesting.com]On Behalf Of K3BU@aol.com
> Sent: Wednesday, October 10, 2001 7:56 PM
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] Re: Force12
>
>
>
> In a message dated 10/10/01 8:21:01 PM Eastern Daylight Time, w8ji@akorn.net
> writes:
>
> >
> > The only mechanism that increases bandwidth is loss in the line.
> > Nothing else.
> > 73, Tom W8JI
>
> Here is one "else":
> Quarter wave "bazooka" coax balun increases the bandwidth. Change in its
> reactance with frequency works "against" reactance change of antenna with
> frequency.
>
> Yuri, K3BU
>
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>
>
>
> List Sponsored by AN Wireless: AN Wireless handles Rohn tower systems,
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> supporting towers up to 100 feet for under $1500!! http://www.anwireless.com
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